1. Field of the Invention
The present invention relates to a voltage source type power converting apparatus for converting power between a multiple-phase AC system and a DC system.
2. Description of the Related Art
In a conventional power transmission and distribution system, a power converting apparatus is disposed at, for example, a 3.3 kV to 500 kV substation or a switching station. The power converting apparatus is operated in a reactive current supply mode or a reactive current consumption so that the transient stability, dynamic stability, static stability, and so forth are improved and hence, the system voltage is maintained constant.
To establish DC transmission or DC linkage between AC systems, a plurality of self-exciting (self-commutating) power converting apparatuses are used so as to control the power interchange and the power flow. Thus, the reliability of the system is improved. In addition, outage propagation can be prevented, and the system reactive power consumption can be reduced.
A power converting apparatus can be disposed in a reception and substation facility for a plant, an electric railway, or the like. The power converting apparatus is operated in a reactive current supply mode and a reactive current consumption mode so as to control the reactive power and stabilize the voltage.
In an electric railway, a power converting apparatus is used for operating electric cars in both a motoring mode and regenerative breaking mode. In addition, in a linear motor railway, the power converting apparatus is used for various control operations.
Furthermore, in general industrial fields, a power converting apparatus is used in a constant-voltage- constant-frequency (CVCF) power supply apparatus so as to maintain the quality of the power supply necessary for operating, for example, a computer.
Conventional circuits for reducing higher harmonics (harmonics neutralization) that adversely affect such power converting apparatuses will now be described with reference to the accompanying drawings.
FIGS. 18(a) and 18(b) show conventional voltage source type power converting apparatuses (hereinafter, "power converting apparatuses"). The circuits include voltage source type power converter units 1a to in (hereinafter, "power converter units"), and phase-shifting transformers 2a to 2n that have phase-shifting primary windings W1a to W1n and secondary windings W2a to W2n, respectively. The phase-shifting primary windings W1a to W1n are connected to AC system side terminals R, S, and T. The secondary windings W2a to W2n are connected to the AC terminals ua to wn of the power converter units 1a to In as shown. DC capacitors 3l to 3n are coupled to the power converter units 1a to 1n, along with DC terminals P and N, as illustrated.
The power converter units 1a to 1n are normally two-level inverters (converters) or three-level inverters (converters) as shown in FIGS. 19(a) and 19(b). DC capacitors 3P and 3N are connected in series to the DC terminals P and N, respectively. Static switches 4a to 4d are also connected in series between terminals P and N. In this circuit, neutral point clamp diodes 4e and 4f are connected together at a point between capacitors 3P and 3N, with the other end of diode 4e being connected at a point between switches 4a and 4b and the other end of diode 4f being connected at a point between switches 4c and 4d.
FIG. 19(c) shows a detailed circuit construction of the static switches 4 and 4a-4d. Each of the static switches 4 and 4a-4d has switch elements with an active switch function (self off switch function) in at least one direction. In other words, the static switch has an active switch element 4" in one direction and a passive diode 4' in the other direction. Examples of the switch element 4", which has the active switch function in at least one direction, are gate turn-off thyristors (such as GTO, MCT, and EST) as shown in FIG. 19(d), or transistors (such as BJT, IGBT, and FET).
Turning back to FIG. 18(b), inverters 1aR to 1nT are single-phase bridge type two-level inverters (converters), or three-level inverters (converters) thereof, as shown in FIGS. 19(a) and 19(b), in which the circuit for one phase is omitted. Transformers 2aR to 2nT are single-phase transformers of which primary windings on the AC system side are connected in series. As another example, a forced commutation thyristor type voltage type power converter can also be used.
Next, operation of the circuits shown in FIGS. 18(a) and 18(b) will be described with reference to a DC-to-three-phase-AC power converting apparatus.
With regard to the circuit shown in FIG. 18(a), the operation when a DC voltage Ed is applied to DC terminals N and P and a three-phase AC is obtained from AC system terminals R, S, and T will be described. In the case of one power converter unit 1a, (6n.+-.1)th order higher harmonics inevitably take place. When n power converter units 1a to 1n with phase differences of 60.degree./n are controlled in six steps and the phases of the primary windings W1a to W1n of the phase-shifting transformers 2a to 2n are shifted against the fundamental wave, higher harmonics of lower than (6n.+-.1)th orders are set off. Thus, the higher harmonics are reduced.
Likewise, in the circuit shown in FIG. 18 (b), the operation when a DC voltage Ed is applied to the DC terminals N and P and a single-phase AC voltage that has been pulse-width modulated (PWM) is generated by each of the inverters 1aR to 1nR, 1aS to 1nS, and 1aT to 1nT will be described. In this example, the phases of triangular carriers that are pulse-width modulated in a-th to n-th inverter units are shifted. The output voltages are achieved by windings connected in series for each phase of the transformers 2aR to 2nR, 2aS to 2nS, and 2aT to 2nT. Since the phases of higher harmonics that take place by switching operation corresponding to the phases of the carriers are also shifted, higher harmonics due to the pulse width modulation are reduced.
As described above, in the circuit shown in FIG. 18(a), to reduce higher harmonics of the composite primary voltage, AC voltages that are output from the power converter units 1a to in with phase differences are composited in series, and the primary windings of the transformers are connected in series. However, since the terminals R, S, and T are connected to the AC system side, it becomes difficult to insulate the series connections of leads (for example, bushings) and the primary windings. In particular, when the AC system uses a relatively high voltage as with a power system, since the insulation grade should be raised, the construction becomes complicated and uneconomical.
In an extreme case, if each of the primary windings (W1a) to (W1n) should be insulated for several hundred kV, the size of the bushings becomes large. To solve this problem, if all portions are housed in the same insulation vessel (a SF6 vessel or an insulation oil tank), the construction becomes impractical with regard to its size, weight, strength, and ability to be transported. To solve this problem, the voltages applied to the terminals R, S, and T are limited to several kV to several ten kV. In addition, an insulation transformer (main transformer) for a hundred and several ten kV or higher is additionally used. In other words, the phase-shifting transformers 2a to 2n cannot be economically connected to the power system.
In the circuit shown in FIG. 18(b), in consideration of the total economical characteristics and availability for an imbalanced three-phase power transmission system, the single-phase power converter units 2aR to 2nR, 2aS to 2nS, and 2aT to 2nT are applied to the three-phase AC system so that each phase of the three-phase AC system can be independently controlled. In this case, to counteract against higher harmonics for each phase, PWM timings (for example, PWM carriers or phases) of individual single-phase power converter units are shifted without the need to shift the phases of the transformers. In addition, the transformers for the phases R, S, and T are housed in the same insulation vessel.
Although an insulation transformer (main transformer) is additionally required, the transformers 2a to 2n for the converters are housed in three insulation vessel and magnetic core portions. The windings and connections are hence unified. However, since the primary windings on the AC system side are connected in series, the currents thereof are the same. Also, to counteract against higher harmonics, since the phases (the PWM timings, for example, PWM carriers) of the single-phase power converter units are shifted, the phase of the voltage of the fundamental wave is shifted. Thus, the active power (thus, a DC side link current) of each of the single-phase power converter units is imbalanced.
Consequently, the DC side terminals P and N must be connected in parallel and cannot be connected in series. In addition, due to this restriction, the DC voltage cannot be raised, for example. Therefore, it is difficult to raise the link voltage to several hundred kV or several ten kV. In other words, although the DC transmission by the voltage source type power converting apparatus provides a good countermeasure against the reactive power, since the total DC voltage should be decreased, the total DC current necessary for transmitting the same power becomes very high (i.e., the long distance transmission line must therefore be large and accordingly, the resistance loss becomes too large). Thus, the apparatus of this type cannot be effectively used.
In the conventional voltage source type power converting apparatus and the power system controlling apparatus, to reduce higher harmonics of multiplexed transformers, anti-power-converter side windings (primary windings) of transformers for converters connected to the multiple-phase AC system are connected in series. Thus, the construction of the transformers for use with the converters becomes complicated.
In addition, in the conventional voltage source type power converting apparatus, when the construction of the transformers for use with the converters is simplified, the power provided by the power converter units become unbalanced. Thus, since the power converter units on the DC link side cannot be connected in series, it is difficult to raise the total DC voltage. Consequently, the apparatus is not suitable for DC transmission. In addition, the capacity cannot be increased in a DC linkage system, reactive power compensation system, and power system control apparatus.
In a power system controlling apparatus that connects anti-power-converter side windings (primary windings connected in series) to a high voltage power system, the windings should be insulated for a high voltage. Thus, an insulation transformer (main transformer) for insulation of a high voltage is additionally required.